Alkaline electrolyzer

Abstract

The invention is directed to a method and electrolytic cell for electrolysis of an alkaline aqueous solution. The method comprises the steps of: - providing an electrolyte (1) chamber comprising an alkaline aqueous solution; - providing a first electrode stack and a second electrode stack, both of which are in fluid contact with the alkaline aqueous solution in the electrolyte chamber and comprise a permeable electrode layer comprising catalytically active electrode material (6, 7), and a permeable non-catalytic layer (8) placed on a first face of the electrode layer facing the electrolyte chamber; - applying a potential difference between the electrode layer of the first electrode stack and the electrode layer of the second electrode stack; - flowing the alkaline aqueous solution from the electrolyte chamber through the first and second electrode stack, thereby causing an oxidation reaction at the electrode layer of one of the electrode stacks and causing a reduction reaction at the electrode layer of the other electrode stack; wherein each non-catalytic layer has a permeability of 1.0 × 10-7 m2 or lower, and / or wherein the flow velocity through the first and second electrode stack is 1.0 × 10-4 m / s or lower.

Description

[0001]P136877PC00 Title: Alkaline electrolyzer BACKGROUND OF THE INVENTION The invention is in the field of electrolysis. In particular the present invention is directed to a method and electrolytic cell for electrolysis of an alkaline aqueous solution. Electrolysis can be used to produce hydrogen from water using electric energy. Especially when the electricity is generated from renewable sources, electrolysis can be used to produce hydrogen with a significantly lower emission of greenhouse gases compared to production of hydrogen from fossil resources. Different types of electrolysis processes exist, most notably proton exchange membrane (PEM) electrolysis, in which a proton-exchange membrane is used as electrolyte and in which electrodes are attached to opposing sides of the membrane, and alkaline water electrolysis, in which the electrodes operate in a liquid alkaline solution, for instance an aqueous solution of potassium hydroxide or sodium hydroxide. In alkaline water electrolysis, the alkaline solution acts both as electrolyte and as reactant. In conventional alkaline electrolyzers, the electrodes are separated by a diaphragm, which separates product gases and transports hydroxide ions (OH-) from one electrode to the other. The diaphragm, sometimes also referred to as separator or membrane, is typically a porous sheet with a thickness between 0.02 and 1 cm, that is selectively permeable to ions while acting as a barrier that separates fluid flows on separate sides of the diaphragm. Alkaline water electrolysis has some advantages over PEM electrolysis, for instance that the electrode materials are less expensive, and that the process is less sensitive to the purity of the water and / or electrolyte. However, traditional alkaline water electrolysis also has some drawbacks related to the use of the diaphragm. First of all, the conductivity of the liquid electrolyte is much higher than that of the diaphragm, which leads to significant ohmic losses. In addition, the limited mechanical strength of the diaphragm may result in failure during shut-down and startup, which leads to high maintenance frequency and downtime of the electrolyzer. Furthermore, the diaphragm can typically not withstand high temperatures that would otherwise be favorable for the efficiency of the overall process. Also, the diaphragms are less effective at preventing gas cross-over at low and high current densities and during fast ramping up or down, and therefore limit the operational flexibility of the process. There have been efforts to develop alkaline water electrolysis processes in which instead of using a diaphragm, other approaches are used for preventing gas crossover. Such processes are also referred to as membrane-less electrolysis. WO2019207479 describes a membrane-less electrolyzer with permeable electrodes. Non-conductive permeable layers are applied to the electrodes in order to inhibit migration of gas formed at the electrode towards the electrode gap. However, the size of the electrodes is limited in view of the maximum electrolyte path length, and in order to scale-up the electrode, multiple ring-shaped bands are used, which presents technical challenges. Other downsides when scaling up the electrolyzer are that the electrode gap is relatively large, resulting in increased ohmic losses in the electrolysis process, which result in a low efficiency. Furthermore, the electrolyzer will become more bulky. In addition, the high flow rate through the electrode also presents difficulties, especially when scaling up the system. WO2022195110 describes electrolyzers for performing electrolysis of water under supercritical conditions. An object of the present invention is to provide alkaline water electrolysis while addressing one or more problems encountered in the art. Another object of the invention is to limit gas cross-over between the electrodes in alkaline water electrolysis. Another object of the invention is to enable (cost)efficient and / or scalable alkaline water electrolysis. BRIEF SUMMARY OF THE INVENTION The inventors had the judicious insight that one or more of the objects can be achieved using an alkaline water electrolyzer in which permeable electrodes are equipped with a non-catalytic layer with low permeability, thereby operating the electrolyzer with a relatively low flow velocity and relatively high pressure-drop over the permeable electrodes. Therefore, according to a first aspect of the invention there is provided a method of electrolyzing an alkaline aqueous solution, comprising the steps of: - providing an electrolyte chamber comprising an alkaline aqueous solution; - providing a first electrode stack and a second electrode stack, both of which are in fluid contact with the alkaline aqueous solution in the electrolyte chamber and comprise a permeable electrode layer comprising catalytically active electrode material, and a permeable non- catalytic layer placed on a first face of the electrode layer facing an internal volume of the electrolyte chamber; - applying a potential difference between the electrode layer of the first electrode stack and the electrode layer of the second electrode stack; - flowing the alkaline aqueous solution from the electrolyte chamber through the first and second electrode stack, thereby causing an oxidation reaction at the electrode layer of one of the electrode stacks and causing a reduction reaction at the electrode layer of the other electrode stack; wherein each non-catalytic layer has a permeability of 1.0 × 10-7m2or lower, and / or wherein the flow velocity through the first and second electrode stack is 1.0 × 10-4m / s or lower. Without wishing to be bound by theory, the inventors believe that the combination of the non-catalytic layer and the pressure drop caused by the low permeability of the non-catalytic layer limits gas cross-over between the electrodes. As a result, lower electrolyte flow velocities are required and smaller electrode gaps may be applied. In addition, the higher pressure drop over the electrodes makes it possible to use larger electrodes. According to second aspect of the invention, there is also provided an electrolytic cell for electrolysis of an alkaline aqueous solution, comprising: - an electrolyte chamber having an inlet for introducing a liquid solution, - a permeable first electrode stack and a permeable second electrode stack, both of which are in fluid contact with an internal volume of the electrolyte chamber and comprise a permeable electrode layer comprising catalytically active electrode material, and a permeable non- catalytic layer placed on a first face of the electrode layer facing the internal volume of the electrolyte chamber; wherein the permeability of each non- catalytic layer is 1.0 × 10-7m2or lower. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a graph of the relation between minimum flow velocity through the electrode stack versus current density. Figure 2 shows a graph of operating voltage versus electric current, showing factors that contribute to the operating voltage needed. Figure 3 shows a schematic overview of an alkaline electrolytic cell. Figures 4 and 5 show schematic overviews of different approaches to introduce an alkaline solution into the electrode gap. Figures 6-8 show schematic overviews of systems comprising a stack of alkaline electrolytic cells. DETAILED DESCRIPTION OF THE INVENTION According to a first aspect of the invention there is provided a method of electrolyzing an alkaline aqueous solution. In the method, an electrolyte chamber comprising a liquid solution, in particular an alkaline aqueous solution, is provided. The alkaline aqueous solution may act as reactant, or as electrolyte, or both. Preferably, the alkaline aqueous solution acts as both reactant and electrolyte. The method also comprises providing a permeable first electrode stack and a permeable second electrode stack. The first electrode stack and the second electrode stack each comprise a first face facing the internal volume of the electrolyte chamber and which first face is in fluid contact with the alkaline aqueous solution in the electrolyte chamber. The first face of the first electrode stack and the first face of the second electrode stack may be placed opposite each other, thereby defining an electrode gap in the internal volume of the electrolyte chamber between the first faces of the respective electrode stacks. Each of the electrode stacks comprises a permeable electrode layer comprising catalytically active electrode material, and a permeable non-catalytic layer placed on a first face of the electrode layer facing the internal volume of the electrolyte chamber (i.e., at the first face of the electrode stack). The first electrode stack and the second electrode stack may have the same or different properties. The electrode stack, the electrode layer and the non-catalytic layer are permeable to both gases and liquids. In order to perform electrolysis of the alkaline aqueous solution, a potential difference is applied between the respective electrode layers of the first electrode stack and of the second electrode stack, while flowing the alkaline aqueous solution from the electrolyte chamber, through the first electrode stack and the second electrode stack, thereby causing an oxidizing reaction at the electrode layer of one of the electrode stacks and causing a reduction reaction at the electrode layer of the other electrode stack. Electrolysis of the alkaline aqueous solution typically results in a product mixture, which comprises gaseous products, such as H2 for the electrode at which the reduction reaction takes place (i.e. the cathode) or O2for the electrode at which the oxidation reaction takes place (i.e. the anode), and typically also comprises unreacted aqueous alkaline solution. The product mixtures produced in the first electrode stack and second electrode stack may flow into a first product chamber, and a second product chamber, respectively. Advantageously, gaseous products generated in the oxidation and / or reduction reaction may be separated from unreacted solution and unreacted solution may be recycled to the electrolyte chamber, whereas gaseous products may be collected and / or stored. Separating gaseous products from unreacted solution may be performed by receiving the fluid flow through the electrode stacks into product chambers having two or more outlets, preferably at least a top and a bottom outlet. Under the influence of gravity, bubbles of gaseous reaction products float upwards and can exit the product chamber from a top outlet located in a top region of the product chamber, whereas unreacted liquid solution can exit the product chamber from a bottom outlet in a bottom region of the product chamber. By performing at least part of the separation of gaseous products from unreacted solution in such product chambers, less effort is needed for any downstream separation steps, if such subsequent separation steps are at all necessary. Preferably, the internal volume of one or both product chambers comprises a porous layer which can further promote separation of gaseous products from unreacted solution. Suitable porous layers may for instance be porous transport layers (PTLs) or gas distribution layers (GDLs). In addition, when electrically conductive such a layer can serve to provide an electronical connection across the product chamber, for instance between the electrode stack and a bipolar plate in case the product chamber is placed between the electrode stack and a bipolar plate. It will be understood that one or more of the product chambers, for instance the first product chamber, or the second product chamber, or both the first and second product chamber, can be equipped with two or more outlets as explained above. In case a product chamber has two or more outlets, the different outlets of that product chamber may be identical or they may be different from each other. For instance, a top outlet from which a stream enriched in gaseous products exits the product chamber may have different dimensions than a bottom outlet from which mainly unreacted electrolyte exits. Surprisingly, the inventors have found that by using a non- catalytic layer with low permeability which results in a large pressure drop over the electrode stack, a uniform flow through the electrode can be achieved even when the aqueous alkaline solution flows through the electrode stacks at a low flow velocity. At the same time, gas crossover between the electrodes is prevented. This is unexpected, because it is generally believed, and taught e.g. in WO2019207479, that higher flow velocities are needed in order to prevent gas crossover, and that a large pressure drop over the electrode stack is unwanted. The approach of preventing gas crossover between the electrodes by controlling the permeability of a permeable non-catalytic layer permeability and / or flow velocity of the alkaline aqueous solution is also different from traditionally used diaphragms or membranes, because contrary to traditional diaphragms which are selectively permeable to ions and liquids, but not permeable to gases, the non-catalytic layer is permeable to ions, liquids as well as gases. In addition, in the method according to the invention, the alkaline aqueous solution flows through the non-catalytic layer, whereas in the case of a diaphragm, the flow of alkaline aqueous solution is parallel to the diaphragm. Additionally, the electrode gap can be kept small, which may prevent ohmic losses, especially when the method is performed at larger scales. Another advantage is that the electrodes can be scaled up while maintaining uniform flow rates through the entire electrode stacks. The flow and pressure gradient over the non-catalytic layer can be seen as a Darcy flow, which is described by Darcy’s law: ^^ = −^^ ΔP ^^^^ in which: q represents the fluid velocity in m / s, k represents the permeability of a medium in m², μ represents the dynamic viscosity of the fluid in Pa.s, L represents the thickness of the permeable medium in m, and ΔP represents the pressure gradient over the medium. Besides the pressure gradient Δ^^^^^^^^over the non-catalytic layer, the electrolyte flow also experiences a pressure gradient Δ^^^^when it flows in radial direction in the electrode gap between the electrode stacks. Preferably, in order to achieve efficient and stable operation of the electrolyzer, the flow through the entire electrode layer comprising catalytically active material is uniform. The resistance to a flow path is expressed as the pressure drop over the path length. From the fluid dynamic requirements that a flow must always follow the path of least resistance it becomes clear that the pressure drop over the non-catalytic layer must be larger than the pressure drop over the length of the electrode gap in order for the electrolyte to reach the end of the electrode gap. In order for the electrolyte to reach every part of the electrode’s surface, the pressure gradient over the non-catalytic layer should be larger than the pressuregradient of the electrode gap, i.e. Δ^^^^^^^^ > Δ^^^^. Preferably, in order to achieveuniform flow through the electrode, Δ^^^^^^^^ > x Δ^^^^, wherein x is 3 or higher,more preferably 5 or higher, such as 10 or higher. The permeable non-catalytic layer may provide one or more of the following functionalities: - inducing a pressure gradient over the electrode stacks, - enabling a uniform flow distribution over the entire electrode surface area, and / or - preventing electrochemical reactions from occurring on the first faces of the electrode stacks facing the internal volume of the electrolyte chamber and defining the electrode gap. The non-catalytic layer may have a permeability of 1.0 × 10-6m2or lower, preferably 1.0 × 10-7m2or lower, more preferably 5.0 × 10-8m2or lower. The permeability of the non-catalytic layer may even be as low as 1.0 × 10-11m2. A too low permeability of the non-catalytic layer may result in too high pressures having to be applied to achieve the required flow of the solution through the electrodes. Therefore, the permeability of the non- catalytic layer may for instance be in a range of 1.0 × 10-7- 1.0 × 10-11m2, or 1.0 × 10-6- 1.0 × 10-10m2, such as 5.0 × 10-8- 1.0 × 10-9m2. In embodiments, permeability of the non-catalytic layer is measured using a suitable permeameter, such as a gas permeameter or a liquid permeameter. Preferably, the permeability of the non-catalytic layer is measured using ISO 4022. Preferably, the flow velocity of the alkaline aqueous solution through the electrode stacks which can be calculated based on the volumetric flow rate of the solution, the area of the electrodes and the number of electrodes, is 1.0 × 10-3m / s or lower, more preferably 1.0 × 10-4m / s or lower. Low flow velocities are preferred because they result in less turbulence (i.e. a low Reynolds number, therefore a more laminar flow) in the electrode gap, thereby minimizing gas crossover between the electrodes in case any gas produced at the electrode would inadvertently be present in the electrode gap. In addition, high flow velocities increase the energy consumption of the total electrolyzer system and increase the capital needs for bigger pumps, both of which can be a bottleneck when scaling up the alkaline electrolyzers. Depending on the operating conditions such as volumetric flow rate, pressure in the system and pressure drop over the electrode stacks, the flow velocity of the alkaline aqueous solution through the electrode stacks may be as low as 1.0 × 10-7m / s or even lower. However, when flow velocities are too low, flow through the electrode stacks may become less uniform, and gas crossover may occur. In addition, depending on the current density applied to the electrolysis cell, a minimum flow velocity might be required, because the applied current density determines the amount of solution that can react into gaseous products such as hydrogen and oxygen, and thereby sets a minimum for the flow velocity. Figure 1 shows a graph of the calculated relation between the minimum required flow velocity and the current density, assuming a 100% faradaic efficiency. Therefore, for example, the flow velocity of the alkaline aqueous solution through the electrode stacks is in a range of 5.0 × 10-3– 1.0 × 10-7m / s, such as 1.0 × 10-4– 1.0 × 10-6m / s. For water dissociation in an alkaline environment, the equilibrium cell potential at room temperature is theoretically 1.23 V, based on the following redox half-reactions: Anode: 4OH- ^ O2 + 2H2O + 4e- Eanode= 1.23 V – 0.059 (pH) vs. NHE Cathode: Ecathode = 0 V – 0.059 (pH) vs. NHE Thermodynamic properties determine the minimum voltage needed for reaction the reaction to occur. A higher cell voltage than the theoretical voltage needs to be applied in order to compensate for energy losses. Several factors contribute to the operating voltage needed, as illustrated in Figure 2, in which: Vrev= reversible voltage or equilibrium cell voltage ΣΔVel= sum of electrode overpotentials ΔVohm = voltage increase due to ohmic resistance Therefore, in embodiments, the applied potential difference between the respective electrode layers of the first electrode stack and the second electrode stack is 1.23 V or more. In practice the applied potential difference is typically somewhat higher than the theoretical value, in order to compensate for inevitable losses, for instance 1.3 V or more, or 1.5 V or more. Preferably, the applied potential difference is not higher than necessary for the electrolysis reaction to occur. Therefore, in embodiments, the potential difference is 3.0 V or lower, preferably 2.5 V or lower. For instance, the applied potential difference is in a range of 1.4-2.8 V or 1.8-2.4 V. The electrolysis is preferably performed at a current density of 500 mA / cm2or higher, more preferably 1.0 A / cm2or higher, even more preferably 2.0 mA / cm2or higher. The electrolysis may even be performed at a current density of 5.0 A / cm2or even higher, but efficiency tends to decrease at increasing current densities. Therefore, the current density may for example be in the range of 500 mA / cm2– 4.0 A / cm2, or 1.0 – 3.0 A / cm2. In embodiments, the non-catalytic layer may be permanently attached to the electrode, for instance using a permanent adhesive, such that the electrode layer and the non-catalytic layer together form a single part. Alternatively, the non-catalytic layer may remain a separate part that is reversibly attached to the electrode layer, for instance by clamping the layers together. Reversibly attaching the non-catalytic layer to the electrode layer enables convenient replacement of the non-catalytic layer without having to replace the electrode layer as well. In embodiments, the non-catalytic layer is provided as a coating on a permeable substrate. Suitable coating materials include ceramics such as nitrides, carbides or carbonitrides of Ti, Cr, or Zr; and resistive metal alloys like AlCr, AlTi, TiSi. The coating may be applied directly onto the permeable electrode layer and / or onto an additional permeable substrate which facilitates permanent or, preferably, reversible attachment of the non-conductive layer to the electrode layer. The additional permeable substrate may be electrically conductive, for instance a metallic mesh (e.g. nickel, stainless steel, titanium or tungsten), and may therefore, while attached to the electrode layer, form part of the electrode layer. Alternatively, the non-catalytic layer may be provided as a self- supporting sheet that can be placed on the electrode layer. A separate permeable substrate as described above may optionally be used to provide additional mechanical strength to the self-supporting sheet and / or to facilitate attachment to the electrode layer. The permeable non-catalytic layer may comprise one or more polymers, one or more ceramic materials, or combinations thereof. If the non-catalytic layer comprises polymer, the polymer preferably comprises one or more selected from PEEK, polyamide, polypropylene, epoxies, silicone resins, polyacrylate, polymethacrylate (e.g. PMMA), polyvinyl acetate, polystyrene, polyurethane, polysulfone, polyethersulfone, polyphenylene sulfone, polyvinyl neodecanoate, as well as copolymers and combinations thereof. In case the permeable non-catalytic layer comprises ceramic material, the ceramic material is preferably selected from silicates, aluminates, zirconates, titanates, and mixtures thereof. Permeability of the non-catalytic layer can be imparted in different ways, for instance by open areas in the layer, generally referred to as pores or apertures. In embodiments, for instance in case the non-catalytic layer is made of an intrinsically non-permeable material, permeability may be provided by apertures in the layer, such as through-holes or perforations. Alternatively or additionally, the permeability may be provided by a porous mesh, such as a woven mesh or a non-woven mesh. In case the non-catalytic layer is provided as a coating on a permeable substrate, the permeability of the non-catalytic layer may originate from the permeability of the permeable substrate, modified by the applied coating. Factors such as the nature and permeability of the coating material, coating thickness, and / or may further influence the resulting permeability of the non-catalytic layer. The term ‘non-catalytic’, as used herein in the context of the non- catalytic layer, refers to the property that the face of the non-catalytic layer forming the first face of the electrode stack (i.e. the face of the electrode stack facing the internal volume of the electrolyte chamber and defining the electrode gap) is not catalytically active, thereby preventing the formation of gaseous products on the first face of the electrode stack in the electrolyte chamber, more particularly in the electrode gap. Since catalytic activity in electrolysis is provided by the applied potential difference between the electrode layers of the respective electrode stacks, this means that the non- catalytic layer is preferably non-conductive across the thickness of the layer, meaning that the non-catalytic layer prevents any potential difference applied between the electrode layers of the respective electrode stacks to be present between the first faces of the respective electrode stacks defining the electrode gap. The non-catalytic layer may therefore also be referred to as non-conductive layer. An appropriate thickness of the non-catalytic layer may be chosen depending on factor such as the material, permeability, electrode size, etc. Typically, the non-catalytic layer will have a thickness in the range of 1-200 µm. Preferably, the thickness of the non-catalytic layer is 5-150 µm, for instance 10-100 µm, or 20-80 µm. Similarly, the size of pores and / or apertures that may be used to provide permeability to the non-catalytic layer can be chosen depending on factors such as the density of apertures, thickness of the layer, etc. If the size of the pores and / or apertures is too large, the permeability of the non- catalytic layer will be too high. Therefore, preferably, pores and / or apertures that may be present in the non-catalytic layer have an average diameter of 25 µm or lower, preferably 10 µm or lower, more preferably 5 µm or lower, or even 1.0 µm or lower. For instance, the pores and / or apertures may have an average diameter of 100 nm – 5 µm, or 200 nm – 2.5 µm. A wide range of catalytically active electrode materials for alkaline water electrolysis are known and are commercially available, for instance comprising metals such as nickel, stainless steel, titanium, tantalum. In some cases, the metallic electrode may have an additional electrocatalytic coating (e.g. nickel-iron). However, the catalytically active electrode materials are not limited to the examples listed here, and in principle, any electrode material suitable for alkaline electrolysis can be used. The first electrode stack and the second electrode stack may be the same, or may be different, for instance based on the intended use of the respective electrode stacks as cathode or as anode in the electrolysis. Therefore, all details described above regarding the non-catalytic layer and / or the electrode layer may be the same for both electrode stacks, or may be chosen independently for the first electrode stack and the second electrode stack, respectively. In order to reduce the pressure drop of the radial flow between the electrodes, the gap between electrode stacks can be increased. However, a too large electrode gap may result in increased ohmic losses. Therefore, preferably, the electrode gap (i.e., the distance between the opposing electrode stacks) is in the range of 0.05-1.0 mm. The pressure drop of the radial flow between the electrode stacks also depends on the maximumelectrolyte path length, which is determined by the size of the electrodes.Using the method of the invention, it is possible to get uniform flow through the electrode stacks, even when the electrolyte path length in the electrode gap is large, i.e., when large electrodes are used. In embodiments, the surfaces of electrodes through which the electrolyte flows have a geometric area of 2000 mm2or more, preferably 5000 mm2or more, such as 10,000 mm2or more, for instance 2500 mm2– 2.0 m2, and the cross section of the electrolyte stacks may have any convenient shape, for instance circular or square. This area is preferably uninterrupted, meaning that the maximum electrolyte path length (i.e., the maximum path length of electrolyte along a surface of an electrode) may be higher than 25 mm, preferably 35 mm or higher, more preferably 45 mm or higher, such as 50- 1000 mm. Because of the low permeability of the non-catalytic layer and / or the low flow velocity through the electrode stack, it is possible to use such large uninterrupted electrode surfaces and corresponding large maximum electrolyte path lengths without significant loss of efficiency. The alkaline aqueous solution may enter the electrolyte gap from the perimeter of the electrodes flowing towards the center of the electrode gap. By simultaneously introducing the alkaline aqueous solution in the electrode gap from different sides, as shown schematically in figure 4, the electrolyte path length in the electrode gap can advantageously be reduced. Additionally or alternatively, the alkaline aqueous solution can be introduced from a center part of the electrode gap flowing towards the perimeter of the electrodes, for instance using one or more inlets in center parts of the electrodes, which also reduces the electrolyte path length in the electrode gap, as shown schematically in figure 5. Reducing the electrolyte path length in the electrode gap may result in more uniform flow through the electrode stacks, especially at low flow velocities. The maximum electrolyte path length in the electrode gap before permeating through the electrode stack may be 25 mm or larger, such as 30 mm or larger, 50 mm or larger, or 100 mm or larger. The maximum electrolyte path length may be as large as 2 m, but will typically be lower than that. For instance, the maximum electrolyte path length is 25-500 mm, preferably 30-300 mm. The maximum distance between adjacent inlets at the perimeter and / or in center parts of the electrode gap which defines the maximum electrolyte path length, may be adjusted accordingly. Preferably, in order to prevent the permeation from gases through the non-catalytic layer towards the electrolyte chamber, the permeability of the electrode layer is much higher than the permeability of the non-catalytic layer, meaning that the resulting pressure drop over the entire electrode stack will essentially be dictated by the permeability of the non-catalytic layer. The pH of the aqueous solution may comprise bases such as potassium hydroxide and / or sodium hydroxide, for instance in a concentration of 10-50 % by weight relative to the weight of the entire solution. Because of this, the pH of the alkaline aqueous solution may be 10 or higher, such as 12 or higher, or 13 or higher. The dynamic viscosity of the alkaline aqueous solution may be in a range of 0.5-10 mPa.s, more preferably 1-2 mPa.s. Preferably, the dynamic viscosity is measured in accordance with ISO 3219, at a temperature of 20 °C. The gauge pressure in the electrolyte chamber may be in the range of 0-200 bar. Positive gauge pressures may be needed in order to overcome the pressure drop caused by the permeability of the electrode stacks, and higher pressures may result in more efficient operation. Therefore, the gauge pressure in the electrolyte chamber is preferably 0.10 bar or higher, more preferably 1.0 bar or higher, for example 5.0 bar or higher. On the other hand, very high gauge pressures may cause formation of gaseous products in the form of microbubbles, which are more difficult to separate from unreacted alkaline aqueous solution than larger bubbles. Therefore, the gauge pressure is preferably 200 bar or lower, more preferably 150 bar or lower. For instance, the gauge pressure in the electrolyte chamber may be 0.1.0-150 bar, or 1.0-100 bar, preferably 5.0-50 bar. The temperature in the electrolyte chamber may be in the range of 5-200 °C, such as 30-150 °C. Higher operating temperatures can lead to improved kinetics of the reactions at the electrodes and reduced ohmic losses, e.g. because formed product gas bubbles are more easily separated from the electrodes. Therefore, the temperature in the electrolyte chamber is preferably 20 °C or higher, more preferably 25 °C or higher, or even 50 °C or higher. On the other hand, too high temperatures may lead to excessive stress on the equipment. Therefore, the temperature in the electrolyte chamber is preferably 200 °C or lower, preferably 150 °C or lower, or even 100 °C or lower. At temperatures and pressures as discussed above, operation of the electrolyzer is intrinsically safer than at higher temperatures and pressures (e.g. supercritical conditions). Another advantage compared to higher temperatures and pressures is that at these conditions, electrolyzers are more suitable for intermittent operation. In addition, at these conditions the equipment may be easier to install and less susceptible to failure, material degradation or wear. Preferably, the volume ratio of gas : liquid in the product mixtures is 1 : 1 or more, preferably 2 : 1 or more, more preferably 3 :1 or more, such as 5 :1 or more. Due to the stoichiometry of the reaction, the gas : liquid ratio will typically be higher at the cathode where hydrogen is formed compared to the anode. The desired ratio between gas and liquid in the product mixtures can be achieved by choosing the flow velocity based on the applied current density, as explained above. The above ratios enable efficient gas-liquid separation. Higher flow velocities might result in lower gas : liquid ratios in the product mixtures, which could result in the formation of microbubbles which are difficult to separate from unreacted alkaline aqueous solution. According to a second aspect, there is also provided an electrolytic cell for electrolysis of an alkaline aqueous solution. The electrolytic cell comprises an electrolyte chamber having an inlet for introducing a liquid solution therein. The electrolytic cell further comprises a permeable first electrode stack and a permeable second electrode stack, and both the first electrode stack and the second electrode stack have a first face that faces the internal volume of the electrolyte chamber. Both the first electrode stack and the second electrode stack comprise a permeable electrode layer comprising catalytically active electrode material, and a permeable non- catalytic layer placed on a first face of the electrode layer facing the internal volume of the electrolyte chamber. The electrolytic cell is preferably suitable for being used in a system as described below, comprising a stack of two or more of said electrolytic cells and in which bipolar plates are used to provide an electrical connection between the electrolytic cells in the stack. This makes it possible to readily upscale and / or downscale an electrolytic system by adding or removing electrolytic cells, without having to make extensive modifications to the electrolytic cell itself or changing the reaction conditions. Preferably, the first face of the first electrode stack and the first face of the second electrode stack are placed opposite each other, thereby defining an electrode gap between the first faces of the respective electrode stacks in the internal volume of the electrolyte chamber. In case the first faces of the respective electrode stacks are oriented parallel to each other, the electrode gap (i.e., the distance between the first faces of the respective electrode stacks) is constant along the electrodes. Preferably, the electrode stacks are substantially planar. This allows multiple electrolytic cells to be conveniently stacked to form a stack of electrically connected electrolytic cells, by using bipolar plates to provide an electrical connection between the electrolytic cells in the stack. By increasing or reducing the number of electrolytic cells that are stacked, a system of electrolytic cells with planar electrode stacks can be scaled easily. Contrarily, for electrolytic cells in which the electrode stacks are curved or annular, stacking of multiple electrolytic cells is typically more complicated, or even impossible, such as in cases where the surface of the electrodes is cylindrical. The electrolytic cell may further comprise a first product chamber configured to receive a fluid flow from the electrolyte chamber through the first electrode stack and / or a second product chamber configured to receive a fluid flow from the electrolyte chamber through the second electrode stack. The first product chamber and / or the second product chamber can each be provided with two or more outlets located in opposite regions of the product chamber. When in use at least one of the two or more outlets is located in a top region, e.g. at the top, of the product chamber, gaseous reaction products can float upwards and exit the product chamber from this outlet, whereas unreacted solution can exit the product chamber from the outlet at in an opposite region (i.e., a bottom region such as at the bottom). In this way, separation of products generated in the oxidation and / or reduction reaction from unreacted solution can at least partly be performed in these product chambers. Because this separation relies on gravity and is passive, less effort and / or energy may be needed for any downstream separation steps, if such subsequent separation steps are at all necessary. Moreover, by providing an outlet in a top region of the product chamber, gaseous products can easily leave the product chamber, thereby preventing excessive pressure buildup in the product chamber which would increase the risk of gas crossover between the electrodes, which is a known problem for traditional alkaline electrolyzers. In addition, overall mass transport is improved by reducing the extent of flooding of the product chamber with electrolyte. This helps with minimizing the entrapment of gas bubbles at the back side of the electrodes which could otherwise reduce the exposed active area of the electrodes and could lead to loss of efficiency. In case a product chamber has two or more outlets, the different outlets may each individually be optimized in terms of dimensions, shape, etc., depending on their purpose. For instance, an outlet which in use is located in a top region of the product chamber, may be optimized for transport of a gaseous product stream and may therefore have different dimensions than an outlet which in use is in a bottom region of the product chamber, and from which mostly liquid will exit the product chamber. Preferably, the product chambers are electrically connected to their corresponding electrode stack, as well as to the bipolar plate in case bipolar plates are used. In this way, the product chambers can conveniently be placed between the electrode stack and the bipolar plate, forming an electrical connection between the bipolar plate and the electrode stack. To this end, the internal volume of the product chambers may comprise an porous electronically conductive layer, for instance a porous metal layer. Suitable porous electronically conductive layers may for instance be porous transport layers (PTLs) or gas distribution layers (GDLs) which are often applied in membrane electrolyzers. An additional advantage of providing such a porous electronically conductive layer, for instance a porous transport layer, in the internal volume of a product chamber is that it allows improved separation of product gas bubbles and unreacted liquid electrolyte. This is especially advantageous when a product chamber has two or more outlets as described above, e.g. in a top region and in a bottom region of the product chamber. Preferably, the electrolytic cell is suitable for carrying out the method of electrolyzing an alkaline aqueous solution as described herein, and it will be understood that, where applicable, embodiments and details described above for the method apply analogously to the electrolytic cell. In figure 3, a schematic overview of an electrolytic cell is provided, showing an electrolyte chamber comprising an inlet (2) for introducing a liquid solution. A permeable first electrode stack and a permeable second electrode stack, comprising a permeable electrode layer (6,7) and a permeable non-catalytic layer (8), comprising a first face that faces the internal volume of the electrolyte chamber are placed opposite each other, defining an electrode gap (1) between them. Also shown are first and second product chambers (4,5) configured to receive a fluid flow from the electrolyte chamber through the respective electrode stacks. Bipolar plates (3) may be provided in case the electrolytic cell is used in a stack. The amount of reaction products at the anode and the cathode is directly proportional to the total current through the electrolyzer, but due to ohmic resistance and other constraints, the production capacity of a single cell at an acceptable efficiency is limited. To this end, there is also provided a system comprising a stack of multiple electrolytic cells as described herein. The system comprises two or more bipolar plates that provide an electrical connection between the electrolytic cells. Typically, the bipolar plates electrically connect the anode of one electrolytic cell to the cathode of another electrolytic cell. In order to reduce ohmic losses, the first electrode stack and the second electrode stack are preferably in fluid contact with each other, meaning that they are both in fluid contact with the same body of electrolyte in the electrolyte chamber, preferably not separated by a diaphragm or membrane that is selectively permeable by ions, and impermeable by fluids. Advantageously, the electrolyte chamber, the first product chamber, and / or the second product chamber may be shared between two or more electrolytic cells, thereby making the system easier to scale. Figures 6-8 show a schematic overviews of exemplary systems comprising a stack of alkaline electrolytic cells. In these figures, a stack of three cells is shown, but the number of cells in a stack may vary. Bipolar plates (3) providing an electrical connection between the electrolytic cells are shown. In figures 6 and 8, first and second product chambers (4,5) of each alkaline electrolytic cell in the stack comprise a top outlet (9-1) and a bottom outlet (9-2). In figure 7, it can be seen how electrolyte chamber (1) is shared between the electrolytic cells. In addition, first and second product chambers (4,5) of the respective electrolytic cells are connected by means of shared outlets (9). Figure 8 shows how top outlets (9-1) of the first product chambers (4) can be connected to form shared outlet (12), while top outlets (9-1) of the second product chambers (5) can be connected to form shared outlet (13). Similarly, also the bottom outlets (9-2) of the respective product chambers (4,5) can be connected to form shared outlets (14,15). EXAMPLES Example 1 – manufacturing of a ceramic non-catalytic layer Porous ceramic layers were prepared that were used as non- catalytic layer. The ceramic non-catalytic layers were prepared by preparing a composition comprising 50 wt.% clay comprising silicate and aluminate (Goerg & Schneider type 264) by weight relative to the total weight of the composition, 27.5 wt.% sugar syrup (also known as ‘golden syrup’, produced by partial acid hydrolysis of a sucrose solution, 85-90% solid content), 8.5 wt.% binder (Zschimmer & Schwarz, Optapix AC112), and 14 wt.% of thickener (Zschimmer & Schwarz, Produkt KM6103). To this composition 1 wt.% antifoam agent (Zschimmer & Schwarz, Contraspum K1012) was added. The composition was spread out in thin layers of ca.1-2 mm, and the layers of ceramic were air-dried. The ceramic was further reduced to a thickness of ca.20-300 µm using a pressure roller. Subsequently, a nickel mesh (US Mesh size 350) was applied as permeable substrate to one side of the ceramic, and the non-catalytic layer was obtained by firing at a temperature of 1000 ^C. The incorporated nickel mesh facilitated reversible attachment of the non-catalytic layer to the electrode stack by clamping. The permeability for gases and liquids of the non-catalytic layer was tested in both directions, and showed that the layer was permeable to both gases and liquids in both directions. This indicates that any prevention of crossover of the gases during electrolysis in the experiments below is caused by flow characteristics of the system (e.g. caused by the permeability of the non-catalytic layer), which is different from a selective membrane in which gas crossover is prevented because the selective membrane is selectively permeable to ions and impermeable to gases. Example 2 – alkaline electrolysis Electrode stacks with different non-catalytic layers were used in an alkaline electrolyzer cell. In all experiments, two porous electrode stacks were placed opposite and parallel to each other in an electrolyte chamber, defining an electrode gap between them. The electrode layers comprised a stack of metallic meshes. The non-catalytic layers were clamped to the electrode layers on the side facing the electrolyte chamber. For the experiments, typically, square electrodes of 50 × 50 mm were used. The body holding the electrodes and non-catalytic layer was transparent, which allowed accurate monitoring of the electrolyte flow through the electrode stacks and gas production. The electrodes were placed such that the electrode gap was oriented vertically and the flow through the electrode stack occurred horizontally. A constant flow of electrolyte was introduced in the electrode gap from the bottom of the cell. During the experimentation, constant flow of electrolyte was provided using a syringe pump. Example 2.1 – ceramic non-catalytic layer. The permeability of the ceramic non-catalytic layer was calculated to be between 5.6 × 10-9and 1.2 × 10-8m2. This permeability resulted in the entire electrode gap to be filled with electrolyte and in an even distribution of electrolyte flow over the entire surface area of the electrode stacks. Flow velocities of 3.3 × 10-7- 8.3 × 10-6m / s were tested. For each of these velocities, a homogeneous flow through the electrode stacks was observed. Furthermore, no gas crossover was observed during operation using current densities up to 2 A / cm2. Example 2.2 – polymer non-catalytic layer. Electrolysis was also performed using nylon woven meshes having a thickness of 45 µm and 60 µm and pore diameters < 1 µm and 50 µm, respectively. The meshes with a pore size of < 1 µm were able to prevent gas crossover at a flow velocity of 2 × 10-5m / s. However, the mesh with a pore size of 50 µm caused such severe gas crossover at a flow of 1.7 × 10-3m / s that the test could not be completed for safety reasons. Example 2.3 – without non-catalytic layer. Without a non-catalytic layer, it was not possible to fill the entire electrolyte chamber using flow velocities up to 1.0 × 10-2m / s, since the total permeability of the electrode stack was too high. This shows that a low permeability of the non-catalytic layer ensures a uniform flow through the electrode stack at low flow velocities.

Claims

Claims1. A method of electrolyzing an alkaline aqueous solution,comprising the steps of: - providing an electrolyte chamber comprising an alkaline aqueous solution; - providing a first electrode stack and a second electrode stack, both of which are in fluid contact with the alkaline aqueous solution in the electrolyte chamber and comprise a permeable electrode layer comprising catalytically active electrode material and a permeable non-catalytic layer placed on a first face of the electrode layer facing an internal volume of the electrolyte chamber, wherein the gauge pressure in the electrolyte chamber is in a range of 0-200 bar and the temperature in the electrolyte chamber is in a range of 5-200 °C; - applying a potential difference between the electrode layer of the first electrode stack and the electrode layer of the second electrode stack; - flowing the alkaline aqueous solution from the electrolyte chamber through the first and second electrode stack, thereby causing an oxidation reaction at the electrode layer of one of the electrode stacks and causing a reduction reaction at the electrode layer of the other electrode stack; - optionally, separating gaseous products generated in the oxidation and / or reduction reactions from unreacted solution; - optionally, recycling unreacted solution to the electrolyte chamber; - optionally, collecting gaseous products; wherein the non-catalytic layer of the first electrode stack has a permeability of 1.0 × 10-7m2or lower, and / or wherein the flow velocity through the first electrode stack is 1.0 × 10-4m / s or lower, and wherein the non-catalytic layer of the second electrode stack has a permeability of 1.0 × 10-7m2or lower, and / or wherein the flow velocity through the second electrode stack is 1.0 × 10-4m / s or lower.

2. The method according to claim 1, wherein the first and secondelectrode stacks are substantially planar.

3. The method according to claim 1 or 2, wherein separatinggaseous products from unreacted solution comprises receiving the fluid flow through the first stack into a first product chamber and the fluid flow through the second stack in a second product chamber, wherein the first and / or second product chamber are provided with a top outlet located in a top region of said product chamber from which a gaseous reaction product exits the product chamber, and a bottom outlet in a bottom region of the product chamber from which unreacted solution exits said product chamber.

4. The method according to any one of claims 1-3, wherein the firstand / or second electrode layer have a geometric surface area of 2000 mm2or more.

5. The method according to any one of claims 1-4, wherein themaximum electrolyte path length is higher than 25 mm, preferably 50 mm or higher.

6. The method according to any one of claims 1-5, wherein the non-catalytic layer of the first and / or second electrode stack has a permeability of 5.0 × 10-8m2or lower.

7. The method according to any one of claims 1-6, wherein the flowvelocity of the alkaline aqueous solution through the first and / or second electrode stack is 1.0 × 10-4m / s or lower.

8. The method according to any one of claims 1-7, wherein the non-catalytic layer of the first and / or second electrode stack comprises one ormore polymers, one or more ceramic materials, or combinations thereof.

9. The method according to any one of claims 1-8, wherein the non-catalytic layer of the first and / or second electrode stack has a thickness of 1-200 µm.

10. The method according to any one of claims 1-9, wherein the non-catalytic layer of the first and / or second electrode stack comprises a porous structure.

11. The method according to any one of claims 1-10, wherein thenon-catalytic layer of the first and / or second electrode stack comprises pores and / or apertures having an average diameter of 25 µm or lower, preferably 10 µm or lower.

12. The method according to any one of claims 1-11, wherein thenon-catalytic layer of the first and / or second electrode stack is provided as a coating on the electrode layer.

13. An electrolytic cell for electrolysis of an alkaline aqueoussolution and suitable for being used in a system comprising a stack of two or more of said electrolytic cells according to any one of claims 20-22, comprising: - an electrolyte chamber having an inlet for introducing a liquid solution, - a permeable first electrode stack and a permeable second electrode stack, both of which are in fluid contact with an internal volume of the electrolyte chamber and comprise a permeable electrode layer comprising catalytically active electrode material, and a permeable non-catalytic layer placed on a first face of the electrode layer facing the internal volume of the electrolyte chamber, wherein the first electrode stack and second electrode stack are preferably substantially planar- a first product chamber and a second product chamber configured to receive a fluid flow from the electrolyte chamber through the first electrode stack and the second electrode stack, respectively; wherein the permeability of each non-catalytic layer is 1.0 × 10-7m2or lower.

14. The electrolytic cell according to claim 13, wherein the first andsecond electrode stacks both have a first face facing the internal volume of the electrolyte chamber, and wherein the respective first faces of the first electrode stack and the second electrode stack are placed opposite each other, thereby defining an electrode gap in the internal volume of the electrolyte chamber between the respective first faces.

15. The electrolytic cell according to claim 14, wherein the electrodegap is in a range of 0.05-1.0 mm.

16. The electrolytic cell according to any one of claims 13-15,wherein the first and second electrode stacks are oriented parallel to each other.

17. The electrolytic cell according to any one of claims 13-16,wherein the first product chamber and / or the second product chamber comprise two or more outlets located on opposite regions of said product chamber.

18. The electrolytic cell according to any one of claims 13-17,wherein the first electrode stack and the second electrode stack are in fluid contact with each other, preferably not separated by a selectively permeable diaphragm or membrane.

19. The electrolytic cell according to any one of claims 13-18,suitable for carrying out the method according to any one of claims 1-12.

20. System comprising a stack of two or more electrolytic cellsaccording to any one of claims 13-19, the stack further comprising bipolar plates providing an electrical connection between the electrolytic cells.

21. System according to claim 20, wherein the electrolyte chamber isshared by two or more electrolytic cells of the stack.

22. System according to claim 20 or 21, wherein one or more of theoutlets of the first product chamber and / or the second product chamber of two or more electrolytic cells of the stack are connected.

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